讯雷下载的核心思想是校验文件的md5值，两个文件若md5相同则为同一文件。
当得到用户下载某个文件的请求后它根据数据库中保留的文件md5比对出拥有此文件的url，
将用户请求挂接到此url上并仿造一个虚假的断点续传请求，从若干url上将一个正常文件拼接出来。
查了下资料，java也可以实现文件md5校验码获取，代码如下:
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
public class MD5Util {
protected static char hexDigits[] = { ‘0‘, ‘1‘, ‘2‘, ‘3‘, ‘4‘, ‘5‘, ‘6‘, ‘7‘, ‘8‘, ‘9‘,‘a‘, ‘b‘, ‘c‘, ‘d‘, ‘e‘, ‘f‘ };
protected static MessageDigest messagedigest = null;
static{
try{
messagedigest = MessageDigest.getInstance("MD5");
}catch(NoSuchAlgorithmException nsaex){
System.err.println(MD5Util.class.getName()+"初始化失败，MessageDigest不支持MD5Util。");
nsaex.printStackTrace();
}
}
public static String getFileMD5String(File file) throws IOException {
FileInputStream in = new FileInputStream(file);
FileChannel ch = in.getChannel();
MappedByteBuffer byteBuffer = ch.map(FileChannel.MapMode.READ_ONLY, 0, file.length());
messagedigest.update(byteBuffer);
return bufferToHex(messagedigest.digest());
}
public static String getMD5String(String s) {
return getMD5String(s.getBytes());
}
public static String getMD5String(byte[] bytes) {
messagedigest.update(bytes);
return bufferToHex(messagedigest.digest());
}
private static String bufferToHex(byte bytes[]) {
return bufferToHex(bytes, 0, bytes.length);
}
private static String bufferToHex(byte bytes[], int m, int n) {
StringBuffer stringbuffer = new StringBuffer(2 * n);
int k = m + n;
for (int l = m; l < k; l++) {
appendHexPair(bytes[l], stringbuffer);
}
return stringbuffer.toString();
}
private static void appendHexPair(byte bt, StringBuffer stringbuffer) {
char c0 = hexDigits[(bt & 0xf0) >> 4];
char c1 = hexDigits[bt & 0xf];
stringbuffer.append(c0);
stringbuffer.append(c1);
}
public static boolean checkPassword(String password, String md5PwdStr) {
String s = getMD5String(password);
return s.equals(md5PwdStr);
}
public static void main(String[] args) throws IOException {
long begin = System.currentTimeMillis();
//2EA3E66AC37DF7610F5BD322EC4FFE48 670M 11s kuri双核1.66G 2G内存
File big = new File("e:/新建文件夹.rar");
String md5=getFileMD5String(big);
long end = System.currentTimeMillis();
System.out.println("md5:"+md5+" time:"+((end-begin)/1000)+"s");
}
}
转载自：http://blog.sina.com.cn/s/blog_53d3c24a0100mpm1.html